Metal salt hydrogen sulfide sensor

ABSTRACT

A hydrogen sulfide sensor is made from a metal acetate film, such as a thin film of copper acetate, formed on a set of monitoring electrodes, by evaporation of a metal acetate aqueous solution disposed on the electrodes, for detecting a weak gas, such as hydrogen sulfide, carried in a gas carrier, such as a nitrogen carrier, for detecting low concentration of the weak gas, such as ten ppm, when the conductivity of the film changes by several orders of magnitude, that produces a metal sulfide, such as copper sulfide, that is a good electrical conductor at room temperature, for example, as the metal acetate is converted directly to a metal sulfide upon exposure to hydrogen sulfide.

FIELD OF THE INVENTION

The invention relates to the field of chemical sensors for detectinghazardous gases. More particularly, present relates to hydrogen sulfidechemical sensors.

BACKGROUND OF THE INVENTION

Conducting polymers, such as polyaniline, have been widely studied aschemical sensors due to simple and reversible acid doping and basededoping chemistry. Polyaniline is a conducting polymer that has beenwidely studied for electronic and optical applications. Unlike otherconjugated polymers, polyaniline has a simple and reversible acid dopingand base dedoping chemistry enabling control over properties such asfree-volume, solubility, electrical conductivity, and optical activity.One-dimensional polyaniline nanostructures, including nano-wires, rods,and tubes have been studied with the expectation that such materialswill possess the advantages of being low-dimensional systems and organicconductors. The change in conductivity associated with the transitionfrom the insulating emeraldine base to the conducting emeraldine saltform of polyaniline is over ten orders of magnitude. This wide range inconductivity has been utilized to make polyaniline sensors that candetect either acids or bases. Polyaniline is widely studied asconducting polymers because of polyaniline environmental stability andstraightforward synthesis. Polyaniline is a useful material for chemicalsensors because polyaniline conductivity can change in the presence ofdoping and dedoping agents. In the undoped state, insulating emeraldineoxidation, polyaniline is a suitable material for chemical sensorsbecause the conductivity can increase by over ten orders of magnitude onexposure to doping acids. This doping process can be reversed duringdedoping when exposed to bases. Polyaniline is difficult to process fromsolution and much effort has been directed toward the improvement ofpolyaniline solubility. N-methyl-pyrrolidinone (NMP), m-cresol, formicacid, and hexafluoro-isopropanol (HFIP) are among the various solventsthat have been used to process polyaniline. The conductivity of dopedpolyaniline is different for each of these solvents due to thedifference in the reactions between the polymer chain and the solventused. Using NMP, polyaniline tends to coil up, and as a result, has lowsolubility in this solvent. On the other hand, in m-cresol and HFIPpolyaniline adopts an expanded coil conformation leading to highconductivity of about 400.0 S/cm. Among these solvents, HFIP is favoredfor processing polyaniline because HFIP has a low boiling point of 59°C. and can dissolve both the emeraldine salt and emeraldine base form ofpolyaniline.

Polyaniline sensors have been used to detect a number of differentchemical species including hydrochloric acid HCl, ammonia NH₃, organicvapors, and strong reducing agents such as hydrazine. Conventionalpolyaniline films processed from other organic solvents and polyanilinenanofiber films processed from water become more insulating uponexposure to hydrazine. In these cases, hydrazine acts as a strongreducing agent, converting the emeraldine salt form of polyaniline tothe leucoemeraldine oxidation form.

Polyaniline nanostructures have received attention as chemical sensorsdue to their high surface area, porosity, and small diameters thatenable rapid and facile diffusion of molecules and dopants into thenanofibers. Current chemical methods of making polyanilinenanostructures, including tubes, wires, rods, and fibers, which involvecomplex synthetic conditions that require the removal of templates whichleads to lower yields with less reproducibility. A practical bulksynthetic template-free method is capable of producing pure uniformnanofibers with small diameters. The polyaniline nanofibers respondrapidly to organic vapors, reducing agents, strong acids, and strongbases significantly better than conventional polyaniline bulk films inall cases. Other work on nanostructured polyaniline as gas sensors hasalso shown that the creation of nanostructures leads to better gasdiffusion because of small diameters. Polyaniline nanofibers withuniform diameters between 30 nm and 50 nm can be made in bulk quantitiesthrough a facile aqueous and organic interfacial polymerization methodat ambient conditions. Thin film sensors made of polyaniline nanofibershave superior performance in sensitivity and in time response to avariety of gas vapors including, acids such as hydrochloric acid HCl,bases such as ammonia NH₃ and butylamine, and alcohols includingmethanol, ethanol, and propanol. However there are a number of gasesthat cannot be detected by the standard, unmodified forms of polyanilinenanofibers.

High quality polyaniline nanofibers have been produced under ambientconditions using aqueous and organic interfacial polymerization.Polyaniline nanofiber films possess much faster doping and dedopingtimes than conventional cast films and have been used in sensorapplications. The nanofibers have nearly uniform diameters between 30.0nm and 50.0 nm with lengths varying from 500.0 nm to several microns.Gram scale products can be synthesized that contain almost exclusivelynanofibers. The synthesis is based on the chemical oxidativepolymerization of aniline in a strongly acidic environment, withammonium peroxy-disulfate as the oxidant. Instead of using thetraditional homogenous aqueous solution of aniline, acid, and oxidant,the polymerization is performed in an immiscible organic and aqueousbiphasic system, in order to separate the by-products, such as inorganicsalts and oligomers, according to solubility in the organic and aqueousphases. However there are a number of gases that cannot be detected bystandard, unmodified forms of polyaniline nanofibers. These includehydrazine and hydrogen sulfide H₂S.

Detection of hydrazine, monomethyl-hydrazine, and unsymmetricaldimethyl-hydrazine, is important because these chemicals are widely usedas rocket fuels but have low harmful threshold limit values of from 1.0to 10.0 ppb. Hydrazine has also been implicated in terrorist incidents.Previous work on conducting polymer based hydrazine sensors includesusing both polypyrrole and polythiophene as the detecting material.Polythiophene sensors can measure very low concentrations of hydrazine,on the parts-per-billion level, but polythiophene is air sensitive andsubject to degradation when stored at room temperature. Polypyrrolesensors are air stable but have high detection limits of one percent.

Additives have been used in biosensors to increase sensitivity ofpolymers to analytes like glucose, urea, oxygen, and chloride. Forexample, carbon nanotubes have been fluorinated. Single walled carbonnanotubes can be defluorinated with anhydrous hydrazine to producehydrogen fluoride and nitrogen. Hydrazine reacts with fluorinatedalcohols, such as HFIP or hexafluorophenyl-isopropanol (HFPP), andhydrogen fluoride acid is produced. When HFIP is added to aqueoussolutions of hydrazine, there is a strongly exothermic reaction with alarge decrease in pH from 11.0 to 3.0.

Hydrazine is a strong reducing agent and is known to reduce both dopedand dedoped polyaniline from a half oxidized polyaniline emeraldineoxidation state to a fully reduced polyaniline leucoemeraldine state.Reduction by hydrazine causes a conversion of polyaniline emeraldine tothe polyaniline leucoemeraldine insulating state. This transformationleads to an increase in resistance as is observed for both thepolyaniline nanofiber and conventional bulk polyaniline. Becauseleucoemeraldine is electrically insulating, the decrease in conductivityassociated with the conversion change can be used to develop polyanilinehydrazine sensors. Doped polyaniline nanofibers respond to hydrazinewith an increase in resistance and a corresponding change in structurefrom emeraldine salt state to leucoemeraldine. The same type of changeoccurs for conventional polyaniline processed from NMP, but the increasein resistance is much smaller. The direct detection of hydrazine withpolyaniline is possible but the response is disadvantageously small.

Hydrogen sulfide is weak acid that is important to detect because it isa colorless and flammable gas that is heavier than air and has theability to cause lung paralysis and death. Generally, a weak acid has apK_(a) value of less than 10.0. Hydrogen sulfide detection is necessarybecause of the potential use in terrorist attacks. Hydrogen sulfide is atoxic dense gas that has a pungent odor and can be fatal at highconcentrations greater than a hundred ppm. Hydrogen sulfide has apermissible exposure limit of twenty ppm but the human odor threshold isabout five ppb. Odor alone cannot be used as an indicator of exposurebecause the sense of smell is lost upon continuous exposure to hydrogensulfide. As a result, sensitive and reliable hydrogen sulfide sensorsare needed with detection thresholds below five ppb.

Existing hydrogen sulfide detection sensors include conductive metaloxides such as tin oxide and tungsten oxide, but these sensors generallyrequire high temperatures for operation. Paper tapes impregnated withmetal salts are also useful and rely on the reaction of the hydrogensulfide with metal salts. Paper tape sensors disadvantageously need arelatively bulky reader with large power requirements and provide only alimited dynamic range of the measurement. Despite the disadvantages ofimpregnated paper tapes, the tapes utilize an important property ofhydrogen sulfide for detection, namely, the ability to react chemicallywith metals and metal ions. The reaction of hydrogen sulfide with metalions forms metal sulfides that can be used to detect hydrogen sulfideH₂S. Paper tapes along with direct optical methods for sulfidemeasurement, utilize absorption for detection.

Strong acids have the ability to fully dope polyaniline resulting inlarge changes in conductivity. Polyaniline gives a robust response tostrong acids because the strong acids have the ability to fully dopepolyaniline resulting in very large changes in conductivity. However,weak acids, such as hydrogen sulfide, only partially dope the polymerand the response of polyaniline to hydrogen sulfide is reduced. That is,weak acids only partially dope the polyaniline polymer, requiring muchmore weak acid to fully dope the polyaniline. Films of the unmodifiedpolyaniline nanofibers do not significantly change resistance whenexposed to hydrogen sulfide. Metal sulfides, in general, are not goodelectrical conductors except for one, copper sulfide. Hydrogen sulfidereacts with many metal salts in solution to form a metal sulfideprecipitate and a strong acid as the by-product as in the reaction ofH₂S+MCl₂→MS+2HCl, where MCl₂ is a metal chloride salt. Being a weakacid, hydrogen sulfide does not interact with polyaniline significantly.Conventional polyaniline responds to ten ppm of gaseous hydrogen sulfideby a small conductivity change. At room temperature and pressure,hydrogen sulfide dissociates only slightly in the presence of water intoH⁺ and HS⁻ because hydrogen sulfide is a weak acid with a pK_(a)=7.05.Therefore, polyaniline can only be partially protonated by hydrogensulfide with a small decrease in resistance. Both doped and dedopedforms of polyaniline nanofibers have been exposed to ten ppm of hydrogensulfide in a humid environment. Both bulk and nanofiber polyanilinerespond only weakly to hydrogen sulfide with the dedoped form polyanlineresponding slightly better than the doped form. The dedoped nanofiberfilm have only a twenty percent resistance change. The doped nanofiberfilm shows almost no change in resistance upon exposure to hydrogensulfide. The unmodified polyaniline nanofiber films provide poor sensorydetection of hydrogen sulfide. Other weak gases of interest, such ashydrogen cyanide HCN, hydrogen selenide H₂Se, arsine AsH₃, phosphine PH₃and other related hydride molecules are practically undetectable byunmodified forms of polyaniline nanofibers.

Copper salts are known to react with hydrogen sulfide both in solutionand in the solid state. Copper acetate reacts with hydrogen sulfide inwater to produce an insoluble copper sulfide precipitate that is blackin color. Copper acetate also reacts with hydrogen sulfide in organicsolutions to produce organosols. Copper sulfide films have beendeposited using atomic layer deposition from the surface reaction of acopper β-diketonate and hydrogen sulfide. Copper acetate films have alsobeen shown to react directly with hydrogen sulfide to form coppersulfide. Copper acetate films are highly insulating and the ability tomeasure high resistances has been limited.

The conductivity and solubility product constants of metal sulfides areknown. The solubility product constant is a parameter for measuring theaqueous solubility of a sparingly soluble material. When a salt isdissolved in water, the salt dissociates into cations and anions and thesolubility product constant is the product of the combined ionconcentrations. A smaller solubility product constant indicates aless-soluble salt that is therefore more stable. The conductivity (S/cm)and the solubility products (Ksp) of metal sulfides include CoS at5×10⁻⁸ S/cm, NiS at 1×10⁻⁷ S/cm and 1.1 Ksp, PbS at 1×10⁻³ S/cm and3×10⁻⁷ Ksp, HgS at 6×10⁻³ S/cm and 2×10⁻³² Ksp, PdS at 1×10⁻³ S/cm and2×10⁻³⁷ Ksp, and CuS at 10.0 S/cm and 6.0×10⁻¹⁶ Ksp. Palladium sulfidehas the smallest solubility product constant with palladium being themost stable metal sulfide. Nickel sulfide has the largest solubilityproduct constant and is the least stable metal sulfide. Copper sulfideis the most conducting of the metal sulfides. Copper sulfide is known tobe conducting but has not been used as an acid sensing materials.Existing hydrazine and hydrogen sulfide sensors are largely ineffective,insensitive, or expensive in use. Hydrazine and hydrogen sulfide areundetectable by pure conventional dope and dedoped polyaniline. Theseand other disadvantages are solved or reduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a sensor for detecting a weakacid.

Another object of the invention is to provide a hydrogen sulfide sensorusing metal salt.

Yet another object of the invention is to provide hydrogen sulfidesensor using metal acetate, such as copper acetate.

The invention is directed to weak acid sensor using a metal salt. Morespecifically, the metal salt is a form of metal acetate. In thepreferred form, the weak acid is hydrogen sulfide that can be detectedby the sensor using copper salt. The detection of hydrogen sulfide H₂Swith improved sensitivity can be had in the absence of a supportingmatrix, such as polyaniline. In the preferred form, a film of a coppersalt is disposed on electrodes without the supporting matrix, todirectly detect hydrogen sulfide acid H₂S through a reaction with thecopper salt. The reaction produces copper sulfide at room temperaturefor providing improved conductivity change of about eight orders ofmagnitude. The preferred copper salt is copper acetate. The metal saltfilm reacts with hydrogen sulfide to form metal sulfide films directlyat room temperature resulting in very large conductivity changes inducedby only parts per million of hydrogen sulfide. Direct electricalmeasurement of the chemical transformation can be used as a method formonitoring or detecting trace amounts of hydrogen sulfide. Electricaldetection of metal sulfide formation is directly made by monitoring theelectrical conductivity of metal salt film as the film reacts withhydrogen sulfide to form metal sulfides at room temperature. Discoveryis made that copper acetate can be directly used to detect trace amountsof hydrogen sulfide. The sensor can be fabricated as a small andsensitive sensor based on direct electrical measurements at roomtemperature. These and other advantages will become more apparent fromthe following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a chemical reaction of copper acetate and hydrogensulfide.

FIG. 2 depicts a process flow for making a hydrogen sulfide sensor usingcopper acetate.

FIG. 3 depicts a hydrogen sulfide sensor using a metal acetate film.

FIG. 4 is a graph showing responses to hydrogen sulfide to metal acetatefilms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, copper acetate Cu(CH₂COO⁻)₂ reacts with hydrogen sulfide acidH₂S to form copper sulfide and acetic acid HOOCCH₃. Copper sulfide isconducting. When a copper acetate film is exposed to hydrogen sulfide, aweak acid, the conductivity of the film changes. As such, copper acetatefilms can be used as a hydrogen sulfide detector.

Referring to FIGS. 1, 2, and 3, and more particularly to FIGS. 2 and 3,a hydrogen sulfide sensor is made by producing 10 a metal salt, such ascopper acetate, in solution and depositing and drying 12 the copperacetate on a sensor substrate. In practice, when the sensor is exposed14 to hydrogen sulfide, a resistance monitor can be used to measure thechange in resistance for detecting the presence of hydrogen sulfide. Apair of electrodes is disposed on the sensor substrate and is used tomake electrical contact with the film. The film is a metal salt film,and is preferably a metal acetate film M²⁺ (RCOO⁻)₂ consisting of ametal M²⁺ and an acetate (RCOO⁻)₂ where R is a radical that ispreferably CH₃CH₂ or CH₃. The metal M is preferably selected from ametal group consisting of cobalt Co, nickel Ni, lead Pb, mercury Hg, andcopper Cu. The weak acid includes hydride molecules that do not dopeunmodified forms of polyaniline films. The weak acid is preferablyselected from a weak acid group consisting of hydrogen sulfide H₂S,hydrogen cyanide HCN, hydrogen selenide H₂Se, arsine AsH₃, and phosphinePH₃.

Referring to all of the figures, and more particularly to FIG. 4, mostmetal acetate films experience a dramatic reduction in resistance whenexposed to a weak acid, such as hydrogen sulfide. In an exemplarprocedure, copper acetate is dissolved in water to give a finalconcentration of 0.1 M. A drop of the resulting solution is then placedon a set of interdigitated gold electrodes on the substrate so as toform a thin film of copper acetate on the substrate. The sensor may be asensor array consisting of multiple sensors. Each sensor may have fiftypairs of conducting figures of the electrodes disposed on a glasssubstrate with gaps of 10.0 μm between the fingers. The resistancechange of a thin film of copper acetate can be measured upon exposure tohydrogen sulfide H₂S at 10.00 ppm at room temperature. The resistance isplotted as R/R₀ where R₀ is the resistance before exposure and R is theresistance after exposure. The real time resistance change of a film ofcopper acetate upon exposure to hydrogen sulfide H₂S is eight orders ofmagnitude.

A flow of a predetermined hydrogen sulfide H₂S concentration isgenerated using a calibrated gas mixture of 200.0 ppm of hydrogensulfide in nitrogen that is diluted with a humidified nitrogen streamusing calibrated mass flow controllers. The humidity can be generatedusing a bubbler and humidity sensor. The copper acetate film respondsquickly and strongly to an exposure of hydrogen sulfide H₂S. The changeof resistance can be eight orders of magnitude in minutes. Of the metalsalts, copper acetate provides the largest response. Copper sulfide isthe preferred conducting sulfide of the expected products from thereaction of hydrogen sulfide with the metal salts. Copper sulfide CuSconducts better than cadmium sulfide CdS, lead sulfide PbS, and zincsulfide ZnS. As such, a copper salt gives a large response relative tothe other metal salts. Some metal salts, such as copper chloride CuCl₂and other related metal chlorides and nitrates do not react withhydrogen sulfide directly in the solid state. However, copper acetateand related salts do react directly to hydrogen sulfide as sensitivematerials for hydrogen sulfide detection. The use of an electrometermonitor with a very large dynamic range and the interdigitatedelectrodes enables monitoring of the resistance changes associated withthe conversion of copper acetate to copper sulfide that is a small bandgap semiconductor with a conductivity of 10.0 S/cm. The sensor does notrequire a high temperature and can be used at room temperature. Thechange in conductivity is rapid with a time response on the order ofτ₉₀=3.8 seconds where τ₉₀ is the response to 90% of full scale. At 100.0ppb of hydrogen sulfide, copper acetate responds with over five ordersof magnitude decrease in resistance with a time response of under oneminute. This large change is attributed to the direct conversion of avery insulating starting material having a high initial resistance to ahighly conducting copper sulfide product having a low final resistance.

Copper acetate is an excellent material for sensing hydrogen sulfidewith much larger changes in conductivity than other copper chloride orcomposites of copper acetate or copper chloride with polyaniline becausecopper acetate films react readily with hydrogen sulfide. Chloride ionsare much more tightly bound to the metal center than acetate ions and,as a result, metal chlorides show no direct response to hydrogensulfide. Unmodified polyaniline nanofibers show no response to hydrogensulfide because hydrogen sulfide H₂S is a weak acid and cannotsufficiently dope unmodified forms of polyaniline when used as a sensor.However, copper acetate is one of several copper salts that do responddirectly to hydrogen sulfide. Other copper salts with large ligands,including copper formate and copper butyrate, respond to hydrogensulfide with resistance changes of several orders of magnitude. Inparticular, copper propionate responds similarly to copper acetate.Carboxylates includes both acetates (CH₃OO⁻)₂ and propionates(CH₃CH₂COO⁻)₂. Other metal acetates including lead Pb, mercury Hg, andpalladium Pd also respond well to hydrogen sulfide forming respectivemetal sulfides after exposure. Silver acetate has a response similar tolead acetate. Salts can be used for detecting other weak acids, forexample, chloroauric acid HAuCl₄ can be used to detect arsine AsH₃. Thedifferences in response of various metal acetates can be attributed tothe initial resistances of the starting materials and the conductivitiesand solubility product constants of the resulting metal sulfides.

The response to hydrogen sulfide is dependant on the solubility productconstant and conductivity of the resulting metal sulfide. Copperresponds the best because copper sulfide has a relatively smallsolubility product constant and a high conductivity. Mercury Hg, leadPb, and palladium Pd sulfides have similar conductivities but respectivesolubility product constants are different. The difference in responseto hydrogen sulfide is related to the difference in respectivesolubility constants. Cobalt sulfide CoS and nickel sulfide NiS areessentially insulating and have high solubility product constants, whichis consistent with the absence of a significant response to hydrogensulfide.

The invention is directed to a simple weak acid sensor using metal saltfilms. The sensor can respond to 100 ppb weak acid gas, or lower. Thesensor is extendable to other toxic weak acid gases such as hydrogencyanide HCN, hydrogen selenide H₂Se, arsine AsH₃, phosphine PH₃, andothers through the formation of other conductive semiconductors fromreactions of metal salts with these gases. Copper acetate films arepreferred for sensitive hydrogen sulfide detection because copperacetate films respond with very large changes in resistance through theformation of a conductive product, such as the copper sulfide product.The response times are on the order of seconds to a couple of minutes atroom temperature. Other copper salts with large, weakly bound ligandsand other metal acetates also respond to hydrogen sulfide by formingmetal sulfides. Those skilled in the art can make enhancements,improvements, and modifications to the invention, and theseenhancements, improvements, and modifications may nonetheless fallwithin the spirit and scope of the following claims.

1. A sensor for detecting a weak acid, the sensor comprising, asubstrate, a pair of electrodes on said substrate, and a metal salt incontact with the pair of electrodes, wherein an electricalcharacteristic of the metal salt changes when the metal salt is exposedto a weak acid, said change in electrical characteristic beingsufficient for display by an external monitor.
 2. The sensor of claim 1wherein, the weak acid is hydrogen sulfide.
 3. The sensor of claim 1wherein, the weak acid is selected from the group consisting of hydrogensulfide, hydrogen cyanide, hydrogen selenide, arsine and phosphine. 4.The sensor of claim 1 wherein, the electrode comprises polyanilinenanofibers and the weak acid comprises hydride molecules that do notdope unmodified forms of the polyaniline nanofibers.
 5. The sensor ofclaim 1 wherein, the metal salt is a metal acetate consisting of a metalion with a +2 valence and an acetate ion having the formula (CH₃COO⁻)₂.6. The sensor of claim 5 wherein, the metal is selected from the groupconsisting of cobalt, nickel, lead, mercury and copper.
 7. The sensor ofclaim 1 wherein, the metal salt is a copper acetate.
 8. The sensor ofclaim 1 wherein, the metal salt is chloroauric acid and the weak acid isarsine.
 9. The sensor of claim 1 wherein, the weak acid is selected fromthe group consisting of hydrogen sulfide, hydrogen cyanide, hydrogenselenide, arsine, and phosphine PH₃, the metal salt is a metal acetate,and the metal is selected from the group consisting of cobalt, nickel,lead, mercury and copper.
 10. A method of forming a sensor for detectingthe presence of a weak acid comprising the steps of, disposing a pair ofelectrodes on a substrate, forming a metal salt solution, placingsufficient metal salt solution on the substrate in contact with each ofthe pair of electrodes, and evaporating the metal salt solution on theelectrodes using dried air.
 11. A method for forming a sensor fordetecting the presence of a weak acid comprising the steps of, disposinga pair of electrodes on a substrate, forming a metal salt solution,placing sufficient metal salt solution on the substrate in contact witheach of the pair of electrodes, and evaporating the metal salt solutionon the electrodes using heated oven air.
 12. The sensor of claim 1wherein, the metal salt is copper formate.
 13. The sensor of claim 1wherein, the metal salt is copper butyrate.
 14. The sensor of claim 1wherein, the metal in the metal salt comprises copper.
 15. The sensor ofclaim 1 wherein, the metal salt is an insulating metal salt convertibleto an electrically conducting material by, a reaction between the metalsalt and the weak acid.
 16. The sensor of claim 15 wherein, a reactionbetween the metal salt and the weak acid to produce a conducting productoccurs at room temperature.
 17. The sensor of claim 16 wherein, themetal salt comprises atoms bound together by large ligands that arebroken by a reaction of the metal salt with the weak acid.
 18. Thesensor of claim 1 wherein, the metal salt is copper acetate, and theweak acid is hydrogen sulfide.
 19. A sensor for detecting hydrogensulfide, the sensor comprising, a substrate, a pair of electrodes onsaid substrate, a metal salt in contact with the the pair of electrodes,the metal salt comprising copper sulfate wherein the electricalresistance of the copper sulfate changes when exposed to hydrogensulfide said change in electrical resistance being sufficient fordisplay by an external monitor.
 20. A method of forming an electrode fordetecting hydrogen sulfide comprising the steps of, disposing a pair ofelectrodes on a substrate forming a copper acetate solution, placingsufficient copper acetate solution on the substrate in contact with eachof the pair of electrodes, and evaporating the copper acetate solutionon the electrodes.